In a disc drive using magnetic recording, data is recorded in concentric tracks on a plurality of surfaces. The recording heads are mechanically ganged together and arranged one per surface with all heads nominally positioned over the same radial track location. The assembly of heads are positioned in unison using a voice coil motor. A servo pattern, consisting of a certain topology of magnetic transitions is recorded on the surface, or surfaces, of the magnetic media. To follow the recorded data, a pre-written servo pattern, containing the recorded data information, is either written continuously along all tracks of one surface (dedicated servo), or is written in small segments of each track on all surfaces (sectored). In either case, the servo information is read back and demodulated to obtain (1) a digital track number, and (2) an analog signal indicating positions from track center. The analog signal indicating position from track center is referred to as the position error signal, referred to also herein as the PES signal. The PES signal may be digitized and manipulated as a discrete number in the disc drive's hardware or software. It is important, especially with multi-gap heads, that the PES signal be substantially linear versus true radial position of the head. With thin-film (TF), single gap read/write heads, it has been customary in practice to only position at the coincident read/write track center, or within .+-.10% of such to recover data. In this case it is important that the small signal gain (PES/True Position) be calibrated to a reference value. For Magneto-Resistive (MR) heads there are two gaps, the MR read element and the TF write element. Because the two gaps are offset, and a rotary actuator will introduce head skew, there can be a considerable displacement between read and write locations. This displacement can be as much as .+-.1/2 track depending on track location and head skew. For this reason, the multi-gap, MR head drive must be able to reliably position over .+-.1/2 track from any track center. In this case it is important that the small signal gain (PES/True Position) be substantially a constant reference value over the .+-.1/2 track displacement. This is another way of stating that PES must be substantially linear versus True Position. See generally, the teachings of U.S. Pat. Nos. 5,473,550 and 5,500,776.
The linearity of the PES signal using prior art patterns and methods is a strong function of the reader micro-track profile, write head geometry and write pattern. Known prior art methods of manipulating PES, such that final PES value is linear versus true position, includes the method of using inverse mapping to linearize the PES signal. The inverse mapping method is also sensitive to the head's magnetic and physical geometry.
It has also been customary practice to use a servo pattern with two channels, normal and quadrature to achieve linearity over 100% of the track pitch. Such servo pattern may consist of the track centers for the normal and quadrature channels each being spaced L tracks apart, and the displacement between normal and quadrature channels being L/2 tracks. The normal and quadrature signals are periodic with length 2L tracks. When the normal channel is zero, the quadrature channel is at an extremma, and vice versa. It is desirable that the point at which quadrature and normal signals are of the same magnitude be equidistant between adjacent normal and quadrature zero points. This point is referred to as the PES commutation point. It is a weakness with prior art servo patterns (especially with MR heads) that the commutation points are asymmetric about the normal and quadrature zero points. This problem can give rise to a discontinuity in the servo position signal at the commutation point, and a resulting servo instability in positioning.
To applicant's knowledge there are no known disc drive apparatus, servo patterns, nor servo pattern generation methods that teach generating a substantially linear position error signal with no commutation asymmetry, nor that teach generating a PES signal that is insensitive to the read head's magnetic and physical geometry's, including the reader micro-track profile.
Thus, a need is seen to exist for a disc drive apparatus, a servo pattern, and a servo pattern generation method for generating a substantially linear position error signal with no commutation asymmetry, and that generates a PES signal which is insensitive to the read head's magnetic and physical geometry's, including the reader micro-track profile.
It is therefore a primary object of the present invention to provide a disc drive apparatus, a servo pattern, and of a triangular wave pattern facilitated by a five (5) discrete level composite W(x), N-Q, according to the principles of the present invention.